1. Field of the Invention
The present invention relates to a positive electrode of a Group III nitride-based compound semiconductor light-emitting device. The term “Group III nitride-based compound semiconductor” refers to a semiconductor represented by AlxGayIn1-x-yN (0≦x≦1; 0≦y≦1; and 0≦x+y≦1) and the same species to which a predetermined additive for determining the conduction type (n- or p-) or for modifying other properties has been added. The Group III nitride-based compound semiconductor also encompasses these semiconductor species in which a Group III element has been partially substituted by B or Tl or a Group V element has been partially substituted by P, As, Sb, or Bi.
2. Background Art
FIG. 4 is a cross-section of a Group III nitride-based compound semiconductor light-emitting device 900. In the Group III nitride-based compound semiconductor light-emitting device 900, one or more n-type AlxGayIn1-x-yN layers 11, a light-emitting layer (active layer) 12 having a single-layer, SQW, or MQW structure, and one or more p-type AlxGayIn1-x-yN layers 13 are formed through, for example, epitaxial growth, on a dielectric substrate 10 such as a sapphire substrate via a buffer layer or a similar layer (not illustrated). After formation of these layers, the n-type AlxGayIn1-x-yN layer 11 is exposed through etching or a similar technique, and an n-electrode 30 is formed on the exposed area. On the p-type AlxGayIn1-x-yN layer 13, a reflecting electrode layer 92 formed from highly light-reflective silver or a silver-base alloy is provided so that the light-emitting device can be operated as a flip-chip-type light-emitting device employing the dielectric substrate 10 side as a light-extraction side. In order to prevent exfoliation of the periphery of the silver or silver alloy reflecting electrode layer 92, the periphery of the reflecting electrode layer 92 is partially covered with a first insulating layer 41 made of silicon oxide. The other portion of the reflecting electrode layer 92 is covered with a thick-film pad electrode layer 94 made of gold or a gold-base alloy. For preventing short circuit and other purposes, the periphery of the n-electrode 30 and that of the thick-film electrode layer 94 are covered with a second insulating layer 42 made of silicon oxide.
Such a technique is widely known in the art, and documents including Japanese Patent Application Laid-Open (kokai) Nos. 2006-41403 and 2006-245232 disclose the technique.
As has been known well, migration of silver ions from the silver electrode layer 92 occurs during passage of electricity. Migration of silver ions tends to occur in the direct formation of the silver electrode 92 on the Group III nitride-based compound semiconductor layer 13. Specifically, when the formed electrode 92 is thermally treated so as to enhance Ohmic contact with the Group III nitride-based compound semiconductor layer 13, the migration may occur. In addition, when silver comes into direct contact with the gold pad electrode 94, inter-diffusion between silver and gold tends to occur during passage of electricity or heating.